CMFileManager: Show loading dialog while loading files

The loading file task costs much time if the folder contains a
lot of files. This causes the illusion of a freeze if the user
presses the back key during this time.

Show a progress dialog while loading and do not allow the
user to press the back key.

CRs-Fixed: 949902
Change-Id: I527d70a55425d20bc50621533fba775bf5d0ead5