Log Message:
gtkmm3: update to 3.24.5.

3.24.5 (stable):

Gtk:
* TreeValueProxy: Mark the copy assignment operator as deleted
  (Daniel Boles) Issue #94, merge request !63

Build:
* Meson build: Use relative paths to untracked/
  (Mingli Yu) Merge request !61
* [gdk|gtk]mmconfig.h.*: Don't dllimport on MinGW
  (Chun-wei Fan) Issue #90 (Lukas K.)
* Meson build: Make it possible to use gtkmm as a subproject
  (Kjell Ahlstedt) Merge request !62 (BogDan Vatra)
* Meson build: No implicit_include_directories
  (Kjell Ahlstedt)
* MSVC build: Gtk::TreeModelColumnRecord: Export class selectively
  (Chun-wei Fan)