Update to Bevy 0.15.0-rc.2

[TimJentzsch]

Objective

Majorly prepares #16.

Bevy 0.15 reworks text and introduces required components. Both of these should simplify how bevy_mod_bbcode works.

We can prepare the update to the final Bevy 0.15 version, by already updating to the current release candidate and fixing the breaking changes.

Solution

• Update to the new hierarchical representation of text. The Bbcode component is now on the same layer as Text and TextSpans are spawned for all parts of the parsed BBcode markup.
• Use required components. Remove BbcodeBundle and just use Bbcode and BbcodeSettings instead.