Add reflowing capability to the text tool.

[epic6666]

• Current behavior

Suppose a single line of text with no line breaks was inserted into a text box. If the text box is resized such that its horizontal dimension is smaller than that of the line of text, the line of text is not reflowed to fit the text box even if enough vertical space to fit it existed in the box.

• Requested behavior

The text tool automatically does its best at fitting a given string of text onto a given textbox size (maybe respecting the fact that it shouldn't split words apart in order to do this, maybe not).

This is basically what the ShareX text tool does.

As discussed in https://github.com/ksnip/ksnip/issues/364