refactor for efficiency
refactor for efficiency

- calculate enable\disable buttons by real width and scroll offset.
- move tooltip calculation to separate function.
- some renaming.

VersifitTechnologies/angular-ui-tab-scroll,m4rcelofs/angular-ui-tab-scroll,m4rcelofs/angular-ui-tab-scroll,tsmes/angular-ui-tab-scroll,tsmes/angular-ui-tab-scroll,VersifitTechnologies/angular-ui-tab-scroll