New format for saving tabs

Don't compress small items and some already compressed formats. This
leads to smaller tab data files and faster loading/saving.

Use qint32 instead int type was used so it's obvious that the data type
is 32 bit (qint32 is guaranteed to be signed int on all platforms
supported by Qt). This means that the tab data files are stored in
platform-independent format (32bit big endian numbers).
Don't check own clipboard data again
Fixes for Windows
Fix icons in items (use only QFont and not CSS)
Use QMimeData method to retrieve some textual formats
Don't use internal owner type outside clipboard monitor
Small widget class for drawing icons
Resize tab tree properly
Ignore empty tab datafile
Hide icon widget if icon is invalid
Center icon widget vertically
Update tab tree size if vertical scrollbar visibility changes
Removed old comment
ItemFakeVim: Updated FakeVim
Create configuration directory before loading any items
Update context menu if first tab is removed
Fixed tests
Tests: Added next/previous command test
Tests: Added some GUI tests
Build fixes