Fixing WiX UI extension screens: texts, icons, margins, spacing

#### Summary of changes:
 - Added a label (in bold) before input fields.
 - Fixed margins and space between elements. There were inconsitencies.
 - Fixed naming inconsistencies in code - e.g., strings in the .wxl for "ServiceCredDlg" started with "ServiceDlg_".
 - Merged into a unique text the 2 "description" labels in DbCreateCredDlg and RuntimeDbCredDlg.
 - Cleaned up unused strings, like texts for 'Browse' buttons that don't exist.
 - Created new error/success icons.

#### Screenshots:
**SelectDbDlg**
![selectdbdlg](https://f.cloud.github.com/assets/4359497/2273791/c094a6fc-9f14-11e3-9471-6992d81314a5.png)
**DbCreateCredDlg**
![dbcreatecreddlg-1](https://f.cloud.github.com/assets/4359497/2273913/f6f485e0-9f15-11e3-80d0-414bc7b74600.png)
![dbcreatecreddlg-2](https://f.cloud.github.com/assets/4359497/2273915/facc3dfc-9f15-11e3-933e-501b1d9f1049.png)
**RuntimeDbCredDlg**
![runtimedbcreddlg-1](https://f.cloud.github.com/assets/4359497/2273927/0b013786-9f16-11e3-8a58-afea0c4b867c.png)
![runtimedbcreddlg-2](https://f.cloud.github.com/assets/4359497/2273929/0f06eb14-9f16-11e3-86b6-adfe82d4e735.png)
**ServiceCredDlg**
![servicecreddlg-1](https://f.cloud.github.com/assets/4359497/2273931/1eb43210-9f16-11e3-9121-56787b257c5b.png)
![servicecreddlg-2](https://f.cloud.github.com/assets/4359497/2273932/2309a1e2-9f16-11e3-9c0b-4012e595088e.png)