Stop build-depending on libgtkmm-2.4-dev

• Explicitly depend on libglib2.0-dev, libsigc++-2.0-dev

  darkradiant explicitly checks for these, so they should be direct dependencies. Adding them will allow the gtkmm2.4 dependency to be dropped.

• Remove unnecessary B-D libgtkmm-2.4-dev

  This is only used in plugins/dm.objectives/util/TwoColumnTextCombo.h, which is not compiled with Debian toolchains (only with MSVC or Xcode).

  Closes: #1041791

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Marked as draft because a test-build hasn't finished yet, but I'm reasonably sure this is right; feel free to merge with appropriate testing.

This change would ensure that darkradiant doesn't get tied up in attempts to remove some of the obsolete GTK-2-related libraries from Debian.

/cc @tobi