RFR: 7056: Fix dark mode [v2]

[Brice Dutheil]

> This change allows a user of the JMC RCP application to switch the appearance with the default Eclipse themes.
> 
> Those are shipped via the `org.eclipse.ui.themes` and `org.eclipse.e4.ui.css.swt.theme` plugin.
> In order to activate them all that is is required is to add the above in the application feature file, and to declare them in the application manifest file.
> 
> 
> 
> **After**
> ![Screenshot 2022-01-16 at 16 23 46](https://user-images.githubusercontent.com/803621/149666554-3016effc-b527-4dd7-aa6b-baa07a9569ad.png)
> ![Screenshot 2022-01-16 at 16 26 21](https://user-images.githubusercontent.com/803621/149666560-cab76fc9-dd9a-4977-9fe7-0a7dc99335c5.png)
> 
> 
> 
> **Before**
> ![Screenshot 2022-01-16 at 16 24 45](https://user-images.githubusercontent.com/803621/149666542-b8e22717-a797-44b6-9805-30b9c14fb9fa.png)
> There's no theme, as such the combo box doesn't open either.

Brice Dutheil has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into rcp-appearance-theme
 - Allow dark appearance for the RCP distribution

-------------

Changes:
  - all: https://git.openjdk.org/jmc/pull/362/files
  - new: https://git.openjdk.org/jmc/pull/362/files/73051261..1c6eb2ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=362&range=01
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=362&range=00-01

  Stats: 57548 lines in 1887 files changed: 31134 ins; 20405 del; 6009 mod
  Patch: https://git.openjdk.org/jmc/pull/362.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/362/head:pull/362

PR: https://git.openjdk.org/jmc/pull/362