Refactor kvstore a bit

* Add debug support (setting in settings:dev)
* When setting a value to void, delete row from database
Drop stray printf()
kvstore: Improve debugging
prop: Add some more helper funcs

prop_copy()
prop_sub_reemit()
prop_get_int()
Improve video playback settings a quite a bit

Most settings (except subtitle sync offset) are stored in a database
for each video file. In addition to that the user can choose to save
the settings as a default for the current folder the video resides in.
This can be very useful for TV series, etc where the user want to
do some specific changes to the playback config for that particular
set of video items.

In addition to this, it's now also possible to apply the current
configuration as the global configuration

Fixes #1836