Add support for egui built-in ctrl+/- and other zoom behaviours

[TeamDman]

Egui provides some built-in zoom factor manipulation.

Egui includes:

• ctrl+'+'
• ctrl+'-'
• ctrl+'0'
• zoom_menu_buttons(ui);

When using the ui.rs example before applying this change, when using the built-in egui scaling hotkeys, the UI flickers for one frame but there is no resulting scale change.

These proposed changes add support for changes to egui zoom_factor being propagated back to the bevy_egui scale_factor.

The changes include adding ctrl+[ and ctrl+] hotkeys to the ui.rs example to demonstrate that both egui and bevy_egui can harmoniously adjust the zoom factor with no one-frame problems.

To experience the changes, run the ui.rs example and use the ctrl+[, ctrl+], ctrl+'+', ctrl+'-' hotkeys to observe scale changes being initiated from both egui and bevy_egui.

It's a bit fiddly to make sure that the values are synced properly while allowing either to initiate a change with no flickering lol

My understanding is that the bevy_egui scale factor was introduced before egui had its own scale factor, so there's future opportunity to remove the bevy_egui scale factor entirely, but until then this should be a good middle ground.

related: #244

[vladbat00]

Hi, thanks for the PR! I'll do a full review a bit later.

My understanding is that the bevy_egui scale factor was introduced before egui had its own scale factor, so there's future opportunity to remove the bevy_egui scale factor entirely, but until then this should be a good middle ground.

You are right. I actually don't mind if we remove it with this PR, might as well make the code simpler.

[TeamDman]

Will give it a shot to remove the bevy_egui scale factor altogether, will make that a separate PR and will abandon this one once ready

[TeamDman]

I tried removing the scale factor entirely, but it wasn't as straightforward as I hoped, will keep this PR open