Enhance popup

[FWDekker]

The current popup would benefit from more customisation and features, including:

• [ ] Putting the settings option at the top, with shortcut 0.
• [ ] If the user presses the 0 mnemonic while focussing on some template, then that template is opened in the settings.
• [ ] The previously selected template is pre-selected when the popup is opened. (Stored by the corresponding template's UUID.) (This does not need to persist between sessions, but could probably made persistent using PropertiesComponent.)

This can be achieved by implementing my own PopupStep.

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Old description: It annoys me to no end that the "0" shortcut in the settings by default refers to the settings, but that's only because it's coincidentally the 10th scheme. I think configuring it to always link to opening the main settings would be a more logical choice.

[FWDekker]

I feel like I've investigated this before, but this is not actually possible, because the numbering is determined by a setting of JBPopupFactory, and is not something I have control over (beyond enabling/disabling it). Therefore, this issue cannot (currently) be solved.

[FWDekker]

Actually, I think I can solve this. Simply put the settings entry at the very top, similar to IntelliJ's own Run configurations popup, where 0 is always the first entry, and pressing 0 while focusing on an entry will open the settings focused on that entry.

[FWDekker]

JBPopupFactory (and the deeply nested class that makes the popup) only supports displaying the entries in the order 1, 2, ..., 9, 0, A, B, ..., Z. As a result, the settings action will be at 1, and the first template action at 2. In the settings dialog, the templates will thus start at 2, which I dislike.

Therefore, the solution will be to implement a custom PopupStep class which has the desired behaviour. With this additional freedom, I can also add other nice features. I will add this to the issue's description. I will postpone the code rewrite to v3.1.0.