Introduce theme addon project template

[glebfox]

For QA:

1. Create a theme add-on project
2. Add any styles, e.g.:

  --primary-color: #0055A6 !important;
  --primary-color_rgb: 0, 85, 166 !important;
  --primary-color-shade-1: #004C95 !important;
  --primary-color-shade-2: #004484 !important;
  --primary-dim-color: #C6D1EC !important;
  --button-primary-focused-border-color: #C6D1EC !important;

3. Publish add-on to .m2 using instructions from README
4. In the Jmix application create a theme extension
5. Add add-on dependency using instructions from README
6. Build and run an app
7. Test that everything works as expected (hint: it may be needed to refresh the page without cache)
8. Close the issue

[knstvk]

Use maven-publish plugin instead of maven. Fix also readme: use publishToMavenLocal instead of install.

[natfirst]

For QA: contact @glebfox before testing