(fix) Multi-track 'Track Properties': accept entered text directly (clearing not required)

[ronso0]

Something I overlooked earlier, it seems:
previously we had to explicitly clear a tag before a manually entered text would be accepted as new text.

background: editors with multiple values have a placeholder text <various>. It is removed when another item is selected (Clear or text item).
On Apply this placeholder is checked to detect whether a multi-value field has been edited.

Now the placeholder is also cleared when the editingFinished() signal is fired (Return, Esc, focus out, checks for valid text).
For the comment editor this signal doesn't exist. There is textChanged() but this is fired for each change (character add/remove), which is not ideal. Instead we catch the focusOut event of the comment editor and check whether the current text is 'new'.

Small fix:
Don't use QString::trimmed() to be consistent with the track table's inline editor.
Only trailing whitespaces are removed, which allows to clear tags by entering a single space.

[cr7pt0gr4ph7]

LGTM, except for the "trim trailing whitespace" code that should really be in an utility function (and should probably handle other types of (white)space instead of just the " ")

[cr7pt0gr4ph7]

This should probably be moved into an utility function mixxx::trimTrailingWhitespace(QString) -> QString in utils/string.h

[cr7pt0gr4ph7]

Small nitpick: comboox -> combobox

[cr7pt0gr4ph7]

Suggestion: // The Clear item is not needed anymore, so remove it.

[cr7pt0gr4ph7]

Suggested change

	inline QString removeTrailingWhitespaces(const QString& str) {
	/// Remove trailing spaces (" ") from the specified string.
	///
	/// Currently only handles simple ASCII spaces (" "). and none of the additional
	/// Unicode whitespace characters (https://en.wikipedia.org/wiki/Whitespace_character#Unicode).
	inline QString removeTrailingWhitespaces(const QString& str) {
	// TODO(XXX): Evaluate whether it makes sense to extend this to all characters within the "Whitespace" Unicode character class

Maybe it would make sense to handle other kinds of whitespace that one may encounter when pasting something from somewhere else? For example, non-breakable spaces.

Although, after checking the sources and against my expectations, QString::trimmed() also only handles the ASCII whitespace characters, and none of the UTF-8/Unicode Whitespace Characters. This is opposed to e.g. .NET String::Trim() which supports all Unicode whitespace characters.

So, probably a topic for a different PR/issue/discussion.