### Problem description
Previous implementation ignored everything after the first newline.
Other changes include:
- Added isEmpty() function that checks if editor has no content.
- replaced asserts with default behavior to avoid unneeded crashes.
- fixed off by one error in DeleteRange()
- some crashes occurred because readily available corrective actions
were not being taken. For example start/end being -1 in DeleteRange().
- At the heart of search/replace is the ability to translate from
indices into the text string to line/column coordinates used for
everything. To this end a function (StringIndexToCoordinates) was added
to do the translation taking utf-8 chars into account. This made the
recently added Utf8BytesToChars function unneeded, so it was removed.
- Removed commented out code that is not useful anymore. Also removed
tooltip code which is also unused.
- Removed unused parameter wrapAround to FindNext().
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