dev-util/geany-plugins: Bump to 1.28 and add gtk3 USE flag

dev-util/geany-plugins: Bump to 1.28 and add gtk3 USE flag

configure decides to build for GTK+ 2 or 3 based on installed geany
using pkg-config.
- Ensure geany and geany-plugins are in step with a gtk3 USE flag (this
  adds unnecessary work for a user not installing any GTK+ using
  plugins, but does no harm)
- Fix GTK+ dependencies to reflect the SLOT that will be used
- Block selection of GTK+ 2 only plugins for gtk3 with REQUIRED_USE
- Explicitly enable all plugins
- Honour LINGUAS, makes it and nl available
- Fix metadata for gtkspell, it is the spellchecker for GeanyVC
- geany-1.26 is the minimum since 1.27
- Prune .la files
- Move to EAPI6, remove use of autotools-utils and versionator, tidy

Gentoo-Bug: https://bugs.gentoo.org/586162
Gentoo-Bug: https://bugs.gentoo.org/566214
Gentoo-Bug: https://bugs.gentoo.org/556840
Closes: https://github.com/gentoo/gentoo/pull/2063

Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>