Be the top handler for .kra files by default

Summary:
Currently, if there are any other apps installed that can open .kra files, and they
have the `InitialPreference` value set in their .desktop file, they will open .kra
files by default instead of Krita. This is non-ideal.

This patch sets the `InitialPreference` value in Krita's own .desktop file high
enough that it will always be the default first handler for .kra files.

CCBUG: 337272

Test Plan:
- Apply the patch and then this one for Gwenview: D19121
- Rebuild and install both apps
- Run `sudo update-desktop-database`
- Right-click on a .kra file in Dolphin. Krita shows up as the top app to open it, with Gwenview showing up in the "Open With" menu

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Subscribers: rempt

Tags: #krita

Differential Revision: https://phabricator.kde.org/D19120