Fixes#8157.
There are some limitations in this dialog as it stands; we will rework
it completely when we revisit BCP 47 tagging.
However, for now, this addresses some strange behaviours around the
Reset button and the displayed output:
* The Reset button will only be enabled if you change the text in the
Language Name field.
* The messaging around a missing language subtag is clearer -- tells
the user that they need to enter something there.
* The Reset button will now never insert default text strings into the
Language Name field.
One example of something that is slightly weird, is if you enter a
two-letter region subtag into the script box, it will show what looks
like a valid tag in the output box (e.g. `en-Au`), but it will be
considered invalid, because that two letter subtag needs to be entered
into the region box.
Updated various places in Keyman Developer to show the full
VersionWithTag rather than the short version number we had been
previously showing:
- About dialog
- Welcome screen
- Installer
- kmcomp help
- kmconvert help
- kmanalyze help
- kmdecomp help
This should help when testing to verify that we are testing the right
version.
Also fixed the keymansentry path when running command line C++ programs
from source repo.