wrappers: add new X-Snap-Exec=$snap.$app support to .desktop files

[mvo5]

Strawman branch based on feedback from seb128 about the challenges of upstream packaging of gtk apps.

This allows upstreams to have a desktop file that contains
both the traditional Exec= line as well as a snap specific way
to start the app. This should make it easy to use the upstream
desktop file directly in meta/gui/.

This allows to write:

[Desktop Entry]
Name=Something
Exec=/usr/bin/non-snap-way to-start-the-thing
X-Snap-Exec=snap.app that-starts-the-thing

[seb128]

thanks for the work, unsure that's the right way though

What Sergio suggested on IRC was to have the declaration in the snapcraft.yaml and let you redefine the exec there

Not having to change the upstream desktop would make things easier

[mvo5]

Closing as requested.