Add "close to the left" and "close to the right" actions for editor tabs

[ShadauxCat]

...mimicking similar functionality in notepad++.

[ShadauxCat]

This is one of the features of notepad++ I use most often in windows environments. After searching through countless linux editors trying to find the same functionality, I finally just gave up and decided to add it myself to notepadqq.

[danieleds]

I love this feature! It's extremely useful.

A few suggestions for the code:

• instead of using a closing flag, check the index of the editor. Pseudocode of what I mean:

  void MainWindow::on_actionCloseRight_triggered() {
      ...
      int currEditorId = m_topEditorContainer->currentTabWidget()->currentIndex();
      m_topEditorContainer->forEachEditor([&](... editorId ...) {
          if (editorId <= currEditorId) {
              return true;
          }
  
          ...
      });
      ...
  }

  Similar thing for CloseLeft(). This simplifies the code and should allow you to reuse it for the two menus instead of replicating it.

• Selecting one of those options should only close the tabs in the current tab widget (not sure if this is the behavior of Notepad++, but I think it's a clearer behavior anyway). So, instead of m_topEditorContainer->forEachEditor(), you might want to do a for loop on m_topEditorContainer->currentTabWidget().

(Regarding the second point, the "close all but this" menu is also wrong... I'll fix it asap).

Ask me anything if you need help! Looking forward for another commit :)

[ShadauxCat]

Sorry for the delay on updating this pull request. Been a busy time lately. But I wanted to drop in and say I'm going to make the requested changes and will try to get a new commit in soon.

[Teklad]

@danieleds or @JuBan1 can this be closed? Its at its 2 year anniversary so I don't see this getting updated by the original author at this point. If one of you guys want you can make the necessary changes and push it to the repo yourself instead if the feature is still desirable.

[ShadauxCat]

@Teklad Ack! Sorry! I forgot this PR existed.... I'll try to update it soon...

[valera-rozuvan]

@ShadauxCat Any progress?

[danieleds]

Done in ac769ea