Avoid disposal of QuickInputExt on hide

[tortmayr]

What it does

Remove implicit dispose() in QuickInputExt.hide(). This prevented quick input/quick pick instances from being shown again after they were hidden. This aligns the behavior with VS Code.
Note that there is still another issue related to this:
The following code sequence will open an empty quickpick overlay:

const quickPick = vscode.window.createQuickPick();
quickPick.items = [{ label: 'A' }, { label: 'B' }];
quickPick.show();
quickPick.hide();
quickPick.show();

This issue is also present in VS Code and is caused by the underlying monaco implementation.
If we also want to fix this we would probably have to directly change this in monaco.
There exists a workaround: Setting the items again before showing results in a correct quick pick overlay.

const quickPick = vscode.window.createQuickPick();
quickPick.items = [{ label: 'A' }, { label: 'B' }];
quickPick.show();
quickPick.hide();
quickPick.items = [{ label: 'A' }, { label: 'B' }];
quickPick.show();

Fixes #13072

Contributed on behalf of STMicroelectronics

How to test

You can use the following plugin (vsix+sourcecode) to test this:
theia13072.zip

This plugin contributed 3 commands

• Test quickpick (hide,show): Creates a quick pick, hides it, then shows it again
• Test quickpick (show,hide,show): Creates a quick pick,shows it, hides it, then shows it again. This is the error case described above that results in an empty quickpick overlay
• Test quickpick (show,hide,show): Same as the show-hide-show command but it also incorporate the workaround (setting items again before showing). This should result in a correct quickpick overlay.

Here is also a quick demo how the commands behave on master and the PR branch:
Master:

quickpick-master.mp4

PR:

quickpick-fix.mp4

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[tsmaeder]

lgtm.