move from appveyor into github actions build

[unitycoder]

• create github actions config (build & release)
• build only main branch or on "#build" message only?
• give automatic version number to release (like appveyor did? or start from same counter value at least?) ** takes current version number from yaml : github.run_number
• remove appveyor files and setup *or can leave as backup
• remove appveyor build status badge
• add github build status badge
• add commits info to build description

[unitycoder]

also would be nice to feed that version number into build (i guess could run script to replace placeholder in some c# script, with that github.run_number value?)
#130

[unitycoder]

added github actions, will look into versio number later

[unitycoder]

actually still need to fix the release infos, should display those commits at least..

now:

---

before: (appveyor)

[unitycoder]

this probably works,
2255357#diff-7829468e86c1cc5d5133195b5cb48e1ff6c75e3e9203777f6b2e379d9e4882b3R44