Various fixes

[Digitalone1]

This PR contains various changes.

The most important is the one about add_new_plugin_to_pipeline function. Many releases ago we introduced the "limiter protection" system that places a new "non-limiter" plugin at the second to last position when the limiter (or the maximizer) is in the last position.

Unfortunately this behavior broke when the level meter was introduced. The level meter does not change the signal, so we put it directly at the bottom of the pipeline (this is the position where the user wants the meter in most cases).

This causes any additional non-limiter plugin to be placed at the bottom, so after the limiter, which is not preferable. I noticed this behavior adding all the plugins at once. In the previous releases I had always a limiter/maximizer at the bottom. Now that the level meter can be added to the pipeline, I don't see the limiter at the bottom anymore (in better words, I see it at the last position until the level meter is added).

This is not the original intended behavior, so I changed the algorithm to check if the level meter in the last position is preceded by a limiter. TLDR: a new non-limiter plugin is added in the third to last position if the level meter at the bottom is preceded by a limiter: new-plugin -> limiter -> level meter. @wwmm please test this behavior.

Then there are some UI changes:

• some plugins had adwpreferences aligned (see Pitch) while others not; now all are aligned;
• the reset history button in Autogain is now inside the Controls group (top space is not wasted);
• the reset history button in Level Meter has been moved at the bottom taking the opportunity to fill the empty space left by the missing reset controls button;
• the Filter had an adwpreferences too long; now it has been split in two groups like some of the other plugins (see Pitch);
• the preset autoload page now has labels for device/preset dropdowns

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