Add a simple popup recurrence picker offering some common recurrence
patterns. Since there is no way to set an end to recurring tasks yet,
this commit also ensures they can at least be deleted.

Co-authored-by: Grena <grena@grenabox.fr>

This commit provides an initial dark theme for the list view and the preferences activity.

Add a initial settings screen to tweak the list view items.

Reduce the number and size of database update operations by being a bit
smarter about when and what to update.

The last refactoring broke the layout of larger cards and cards of
closed tasks. This restores the correct look.

The new version contains a couple of fixes.

Linkification of task description was temporarily removed when the
checklist implementation was improved. This commit introduces a floating
action mode which prompts the user before opening a link. This also
fixes #895.

Improve the list layout by reducing margins and removing obsolete elements and code.
It doesn't quite fix the scrollbar size on tablets for some reason. This needs further investigation.

* Also put Play Store fields under version control.
* Fix build errors.

This will make long checklists on marginally faster. So solve the issue properly we'll have to switch to a `RecyclerView`.

This is an initial redesign and refactoring of the light theme. This also prepares for a future dark theme.
The new theme was separated into a new module so we can share it among modules.

fix android.util.AndroidRuntimeException: Calling startActivity() from outside of an Activity  context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want? (#947)

This adds a few more RowData implementations to support the implementation of SubTasks.

When a child task refers to its parent via the parents UID and the parent is inserted after the child, we need to update the PARENT_ID of the child once the parent is available.

PARENT_ID was meant to serve as a shortcut to parent-child relationships between tasks, for both writing and reading.
While the reading part already works (with the exception of issue #877) the writing part didn't work yet. This is fixed by this commit.

Also show task in widget in red color also if due date is equal to now. Otherwise a task would wrongly remain white if its view is updated exactly at the due date.

Use an important notification to inform the user about a stale list rather than launching an activity from the background.

In addition this changes the check list UI. Instead of splitting the description into
a description and check list part, we treat each paragraph either as text paragraph or check list item. Checkable items and text items can be converted into each other.

Translation correction.

Try to work around the crash caused by an IllegalStateException by always returning a valid group view. Some change in Android seems to cause this. We'll keep this workaround until we've removed the `ExpandableListView`for good.

Update Polish translation.

Translation update.

In order to support simple recurrence models and also keep long running tasks small we detach completed instances at the beginning of a series into separate task instances.

Also update gradle wrapper and travis file and fixes a build error.

The total number of users of Android versions < 4.4 sums up to less than 1% on Google Play. That's not enough to justify supporting these legacy versions anymore. Dropping these, on the other hand, allows us to ignore backwarnds compatibility issues.

Added Danish translation.

The Task Editor used to launch the details view for newly created tasks using the task Uri. In general this works, but it fails when the details view uses the Uri with the notification service (because that currently requires an instance Uri). So to fix this we load the first instance after creating a new task and use that one to show the details view.