Support schemas for layer definition

It extends the "Create layer" feature, allowing an user to load/save a schema (an XML file) which defines the fields (name, type and length) to be created.

![opencadtools-schema](https://f.cloud.github.com/assets/583546/277560/71559708-90ca-11e2-9526-376506216cf0.png)

Highlights:

- When a user loads the schema, the fields are added to the ones already in the table. It allows adding up schemas, use a base schema as template while adding different fields for different layers, etc.
- Save button only enabled when the table has rows.
- Change OpenCADTools to be deployed into lib folder inside gvSIG/extensions/com.iver.cit.gvsig.cad/ just like the libraries used (xstream and smc).
- Tested with Shapefile and PostGIS layers.
- Introduce new strings for internationalization (already translated into english, german, spanish and galician).
- The file browsers for save/load XML file uses "Templates" folder.
- When save the schema, run following validations (see https://github.com/opencadtools/opencadtools/commit/4184060334ea0ff867682c840605793abbf491e0, https://github.com/opencadtools/opencadtools/commit/b464808efadf65098adddc6107a583fe65adaad0, a24983fb359deac3e6748baf06dd79f47f8f8564 and b50089cad842e4ae3b56df1630636df2387bde92):

  - Field name must be unique.
  - Field name must not be empty.
  - Field name must not be longer than the driver capabilities (for ex: 10 characters in Shapefiles) or 254 characters if the driver does not define a max.
  - Field name must not contain whitespaces.
  - Field name must not contain non-word characters (others than "a-z", "A-Z", "0-9" and "_").
  - Field name must not be any of reserved words (now defined as a file with SQL key words, but may be updated by the user to fit her needs - for example, may include other words as "cat" or "_cat" which may cause troubles with GRASS backend in SEXTANTE).
  - Show a warning if field name contains uppercase characters.
  - If field type is "String", field length is truncated to 254 characters if it is longer than that.

- When load the schema, run following validations (see deaba82434e7f402e51b4d361c370b1c1d1f0752):

  - Show the user a message when reading the XML and a parsing error happens (for example: a field definition misses a field tag). See invalid XML files as an example: [misses field tag but have other field definitions OK](https://github.com/opencadtools/opencadtools/blob/deaba82434e7f402e51b4d361c370b1c1d1f0752/data/schema-invalid-some-field-has-half-field-tag.xml), [misses field tag](https://github.com/opencadtools/opencadtools/blob/deaba82434e7f402e51b4d361c370b1c1d1f0752/data/schema-invalid-with-half-field.xml) or [has not any field tag](https://github.com/opencadtools/opencadtools/blob/deaba82434e7f402e51b4d361c370b1c1d1f0752/data/schema-invalid-without-field.xml).
  - Load all field definitions although some of them misses name, type or length. That way, the user can spot easily minor errors in XML (for ex: a field type missing), fixed them and update the XML file.
- Some of the validations did exist already for the creation of LayerDefinition - see [``validateTableModel``](https://github.com/opencadtools/opencadtools/pull/40/files#L14R97). Others were created only for this feature - see [``validateSchema``](https://github.com/opencadtools/opencadtools/pull/40/files#L14R157). Merge them to take all validations into account in both process is easy.