theme: oscar - add accesskey to input tags

l - language field
s - search field
t - time range field

The way to activate the accesskey depends on the browser and its platform [1]

[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/accesskey



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>