Remove some bugs, warnings, and unused code

The first commit is from !88 (closed), forgot I had it on my branch.

This is a bunch of smallish changes that generally remove code or fix bugs:

• Some files, like dummy_dep.h and dialog.h/dialog.c, weren't actually used anywhere, so just remove them.
• A few files, like diaerror.h/diaerror.c and utils.h/utils.c, can be removed with a little effort.
• Assume isfinite is present, fixing #525 (closed).
• Don't use diagtkrc on GTK3, and stop initializing colourmaps altogether. Both don't seem useful on modern systems?
• Fix a handful of memory errors, warnings when you click at precise coordinates, and a typo in the export dialog code.
• Remove some unused build checks.
• Fix some compiler warnings.

It also introduces a new dia-version-info.h, so when the commit changes there's no need to rebuild everything due to config.h changing. I'm not sure I wrangled Meson into doing everything right, but it seems to build and make some sense. layout.cpp stopped building right, so I slightly changed the includes to get it working.

Also: I also have a handful of other, larger/more subjective changes done. Should I make separate MRs for each, put them all together, or organize them some other way?