Refactor receiver handling and update solution structure

- Updated `BaseController.vb` to use `Receiver.Id` for deletion.
- Changed `Receivers` property in `frmEnvelopeEditor.vb` to `BindingList(Of ReceiverVM)` and adjusted initialization and usage throughout the file.
- Modified `frmFieldEditor.vb` to use `List(Of ReceiverVM)` for receivers and updated related methods.
- Adjusted color assignment logic in `frmMain.vb` to utilize a new `Value` class.
- Added new project for `EnvelopeGenerator.Form` in the solution file.
- Introduced new enums `ContractType` and `PageOrientation` in their respective files.

EnvelopeGenerator.Domain/Constants/ContractType.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.Domain.Constants
+{
+    public enum ContractType
+    {
+        Contract = 1,
+        ReadAndSign = 2
+    }
+}

EnvelopeGenerator.Domain/Constants/PageOrientation.cs

@@ -0,0 +1,8 @@
+namespace EnvelopeGenerator.Domain.Constants
+{
+    public enum PageOrientation
+    {
+        Portrait = 0,
+        Landscape = 1
+    }
+}