Added zoom functionality to sprite frames editor plugin

[javidcf]

Zoom buttons and mouse control added to spritesheet preview and animation frames pane.

Solves godotengine/godot-proposals#1278

The zoom functionality is pretty much the same as in TileSetEditor, just the usual buttons plus the Ctrl+scroll mouse control.

It also adds zoom to the frame previsualization panel.

[YeldhamDev]

All those buttons should have flat enabled.
And would be nice for the "Select Frames" one to be inside the frame and to the left, for consistency with other parts of the editor.

[javidcf]

@YeldhamDev Thanks, I've fixed that now. To get the positioning right (without making a whole lot of changes, i.e. replacing a ScrollContainer with a panel with two scrollbars and custom scroll handling, as is done in other cases) I had to use set_h_size_flags(0) / set_v_size_flags(0), not sure if that is supposed to be valid? But there isn't an explicit enum to get that "default" behavior from fit_child_in_rect.

Btw, in TileSetEditor the zoom buttons are not flat, I could make that change too since it's just a couple of lines but I'm not sure it's appropriate to have it in this commit/PR. Also, in that case the zoom buttons are next to an information sign button ("Display Tile Names") which I'm not sure if should be made flat or not.

[YeldhamDev]

not sure if that is supposed to be valid?

As far as I'm aware, yes, that's valid.

Btw, in TileSetEditor the zoom buttons are not flat, I could make that change too since it's just a couple of lines but I'm not sure it's appropriate to have it in this commit/PR.

Yes, a separated PR would be better, and yes, the "Display names" button should be flat. In addition, those buttons should also be made flat:

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.4.

[akien-mga]

Cherry-picked for 3.2.4.

Actually no, there are significant changes between 3.2 and master which make this non-trivial to cherry-pick, so I'm not confident that what I've done would actually work as designed. So a dedicated PR would be welcome if this change should be backported to the 3.2 branch.