Placeholder resolving from environment block

[eoudejans]

Python Subprocess.Popen env variable sets the process environment block. This PR makes sure that placeholders are also resolved based on the environment block, next to known or unknown registry keys. Without this PR the env parameter cannot be used to communicate with geodms, in light of the regression testing rework.

Simple test code to test out interpreting the environment block from c++:

geodms:

container test
{
	parameter<string> test := Expand(.,"%SourceDataDir%"), StorageName="%projDir%/data/test.txt", StorageType="str", StorageReadOnly="False";
}

python:

import subprocess
import os

cmd_parts = ['cmd','/k', 'title', 'test','&&', 'call', 'C:/dev/geodms/geodms_v17/bin/Release/x64/GeoDmsGuiQt.exe','/S1','/S2', '/S3',\
             'C:/Users/Cicada/Desktop/test_subprocess_env/cfg/test_subprocess_env.dms','test']

custom_env = os.environ.copy()
custom_env['SourceDataDir'] = 'I changed the sourcedatadir using the environment block'

parent_process_open_handle = subprocess.Popen(
    cmd_parts,
    cwd=None, env=custom_env, 
    creationflags=subprocess.CREATE_NEW_CONSOLE
)

Output in "%projDir%/data/test.txt" should be equal to custom_env['SourceDataDir'].

[MaartenHilferink]

Note that GetConvertedRegConfigSetting, called at line 408 -> 420 calls GetRegConfigSetting(..., key), which calls PlatformInfo::GetEnvString(Överridable", key), which calls GetEnv("GEODMS_Overridable_"+key); but for SourceDataDir, this is now shielded off by the earlier call (line 403) to GetPlaceholderValue(configDir, "SourceDataDir"), which returns GetSourceDataDir().