qml: cut too long empty indicator texts in media library views

It is planned to disable word wrap in EmptyLabel (!7079 (merged)), in an attempt to have sensible implicit size. This allows us to use minimum width approach instead in the main view.

There is little use for being responsive (via word wrap) in the empty indicator. Usually views should have minimum size which is at least the empty indicator size (similar to the playlist pane). Word wrap does not look good, the words can stack up and down, making a column of many words.

In !7079 (merged), this is especially necessary because MainViewLoader is going to center the empty indicator and not fill the whole view (because if it fills, its potential background would cover the whole view), invalidating its responsive approach (as the implicit size is going to be used instead). With too long text, the text may leak beyond the view borders.

Instead, I propose cutting the text, so that we can have sensible minimum size (width). Currently the window already has reasonable minimum width, and there is also isScreenSmall which makes the playlist pane be placed on top of the main view (which makes the main view have as much width as the window has).

Request review @chub.