git-gui: extract script to generate macOS app

git-gui: extract script to generate macOS app

Extract script to generate the macOS app. This change allows us to reuse
the build logic with the Meson build system.

Note that as part of this change we also modify the TKEXECUTABLE
variable to track its full path. Like this we don't have to propagate
both the TKEXECUTABLE and TKFRAMEWORK variables into the script, and the
basename can be trivially computed from TKEXECUTABLE anyway.

Signed-off-by: Patrick Steinhardt <ps@pks.im>