Update qgit.icns with more sizes

Update qgit.icns with the following icon sizes:
- 16x16
- 16x16@2x
- 32x32
- 32x32@2x
- 128x128
- 128x128@2x
- 256x256
- 256x256@2x
- 512x512
- 512x512@2x

Note that the previous qgit.icns had only a single icon of 64x64
which is not enough to some MacOS versions like 14 (Sonoma).

Signed-off-by: Filipe Rinaldi <[email protected]>