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### Problem description
<!-- Describe the bug that you fixed/feature request that you
implemented, or link to an existing issue describing it -->
Implement a Linux backend for the ProcessMemoryProvider plugin.
### Implementation description
<!-- Explain what you did to correct the problem -->
Most of the provider code is the same between Windows and Linux. The
primary differences are:
- enumerate PIDs in `/proc/` to get the process list
- use `/proc/<PID>/cmdline` as the process name
- parse `/proc/<PID>/maps` to get the module list
- reading/writing from memory is done using
`process_vm_readv`/`process_vm_writev`
NOTE: `sudo setcap CAP_SYS_PTRACE=+eip build/imhex` must be run to give
the binary permission to read another process' memory. Running as root
user should also work but I would not recommend it.
### Additional things
The existing translations keys no longer match since I moved the plugin
from `windows` to `builtin`.
I'm not well versed in C++ so I attempted to keep my changes rather
simple. Feedback is very welcome.
---------
Co-authored-by: WerWolv <werwolv98@gmail.com>
### Problem description
Currently, the providers use the method `isSavable()` to determine both
if they can use "Save" or "Save as".
This behaviour is problematic because some providers may need to be
saveable but not saveable as: for example the view provider. The
original provider may not allow to be saved.
### Implementation description
I separate these two behaviour by creating another function:
`isDumpable()`, that return true by default but can be overridden by the
provider to return false, if the provider should not be dumped in any
way.
### Additional things
While I was at it, I also marked "export" operations as needing the
"dumpable" flag. That way, we can't accidentally export the whole
address space of a process as base64.
I also added documentation for these some functions in Provider
