Added TextWidget set/getPreferredCharacterWidth methods.

These allow a default width for text fields to be specified, while still allowing them to be resized if necessary. Renamed set/getCharacterWidth methods to set/getFixedCharacterWidth and added documentation to clarify their relationship to the new methods. The old methods remain for backward compatibility but are deprecated.
Increased preferred width of PathWidgets.

This increases the size of things like the OpDialogue to a more reasonable size to take into account the long paths that tend to be entered there.

Fixes #515.
Fixed Timeline start/end field sizing bug.

The Timeline itself sets an explicit size for the fields, but this was getting overwritten by the NumericPlugValueWidget every time it updated the value from the plug. Since the max value of a plug is not mutable, it's sufficient for NumericPlugValueWidget to only set the width on construction and in setPlug(), leaving UIs that know better to override the width, safe in the knowledge that it will stay there.

Fixes #111.
Merge pull request #633 from johnhaddon/textFieldWidths

Text field width fixes