win32: Building distribution via gitlab ci

Example pipeline with those changes produces .msi and .zip artifacts for 32-bit and 64-bit mingw - https://gitlab.gnome.org/galkinvv/meld/pipelines/25098

The first commit optimizes the installer build time - it was a bit huge since it copied several thousands of adwaita icons several extra times. This made builds uncomfortable long (4-6 minutes). Now they are done in 1-2 minutes on typical hardware.

The second commit introduces mingw build on gitlab ci.

The resulting installer/zip installs/runs fine and can be used for file/folder comparisons. However, execution of external tools - for version control, for opening help, links on about page doesn't work for some reason.

So, its more functional than current appveyour build of 3.19 which immediately fais on start due to old gtk, but less stable than 3.18.2