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