Fix numeric checking on DTextEntry and children.

Inserting a special character like "%" will be parsed as a pattern and error. So we tell string.find not to use pattern checking.

Fixes: https://github.com/Facepunch/garrysmod-issues/issues/773
Merge pull request #584 from HandsomeMatt/dtextentry_checknumeric

Fix numeric checking on DTextEntry and children.