RFR: 7056: Fix dark mode

[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.

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Commit messages:
 - Allow dark appearance for the RCP distribution

Changes: https://git.openjdk.java.net/jmc/pull/362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7056
  Stats: 14 lines in 2 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/362.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/362/head:pull/362

PR: https://git.openjdk.java.net/jmc/pull/362